New tests.

test/libsolidity/semanticTests/arithmetics/block_inside_unchecked.sol

@@ -0,0 +1,13 @@
+contract C {
+  function f() public returns (uint y) {
+    unchecked{{
+        uint max = type(uint).max;
+        uint x = max + 1;
+        y = x;
+    }}
+  }
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 0x00

test/libsolidity/semanticTests/arithmetics/check_var_init.sol

@@ -0,0 +1,18 @@
+contract C {
+    uint public x = msg.value - 10;
+    constructor() payable {}
+}
+
+contract D {
+    function f() public {
+        unchecked {
+            new C();
+        }
+    }
+    function g() public payable returns (uint) {
+        return (new C{value: 11}()).x();
+    }
+}
+// ----
+// f() -> FAILURE
+// g(), 100 wei -> 1

test/libsolidity/semanticTests/arithmetics/checked_add.sol

@@ -0,0 +1,13 @@
+contract C {
+    // Input is still not checked - this needs ABIEncoderV2!
+    function f(uint16 a, uint16 b) public returns (uint16) {
+        return a + b;
+    }
+}
+// ====
+// ABIEncoderV1Only: true
+// ----
+// f(uint16,uint16): 65534, 0 -> 0xfffe
+// f(uint16,uint16): 65536, 0 -> 0x00
+// f(uint16,uint16): 65535, 0 -> 0xffff
+// f(uint16,uint16): 65535, 1 -> FAILURE

test/libsolidity/semanticTests/arithmetics/checked_called_by_unchecked.sol

@@ -0,0 +1,14 @@
+contract C {
+    function add(uint16 a, uint16 b) public returns (uint16) {
+        return a + b;
+    }
+
+    function f(uint16 a, uint16 b, uint16 c) public returns (uint16) {
+        unchecked { return add(a, b) + c; }
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f(uint16,uint16,uint16): 0xe000, 0xe500, 2 -> FAILURE
+// f(uint16,uint16,uint16): 0xe000, 0x1000, 0x1000 -> 0x00

test/libsolidity/semanticTests/arithmetics/checked_modifier_called_by_unchecked.sol

@@ -0,0 +1,15 @@
+contract C {
+    modifier add(uint16 a, uint16 b) {
+        unchecked { a + b; }
+        _;
+    }
+
+    function f(uint16 a, uint16 b, uint16 c) public add(a, b) returns (uint16) {
+        return b + c;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f(uint16,uint16,uint16): 0xe000, 0xe500, 2 -> 58626
+// f(uint16,uint16,uint16): 0x1000, 0xe500, 0xe000 -> FAILURE

test/libsolidity/semanticTests/arithmetics/signed_mod.sol

@@ -2,6 +2,14 @@ contract C {
     function f(int a, int b) public pure returns (int) {
         return a % b;
     }
+    function g(bool _check) public pure returns (int) {
+        int x = type(int).min;
+        if (_check) {
+            return x / -1;
+        } else {
+            unchecked { return x / -1; }
+        }
+    }
 }
 
 // ====
@@ -12,3 +20,5 @@ contract C {
 // f(int256,int256): -7, 5 -> -2
 // f(int256,int256): -7, 5 -> -2
 // f(int256,int256): -5, -5 -> 0
+// g(bool): true -> FAILURE
+// g(bool): false -> -57896044618658097711785492504343953926634992332820282019728792003956564819968

test/libsolidity/semanticTests/arithmetics/unchecked_called_by_checked.sol

@@ -0,0 +1,17 @@
+contract C {
+    function add(uint16 a, uint16 b) public returns (uint16) {
+        unchecked {
+            return a + b;
+        }
+    }
+
+    function f(uint16 a) public returns (uint16) {
+        return add(a, 0x100) + 0x100;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f(uint16): 7 -> 0x0207
+// f(uint16): 0xffff -> 511
+// f(uint16): 0xfeff -> FAILURE

test/libsolidity/semanticTests/arithmetics/unchecked_div_by_zero.sol

@@ -0,0 +1,22 @@
+contract C {
+    function div(uint256 a, uint256 b) public returns (uint256) {
+        // Does not disable div by zero check
+        unchecked {
+            return a / b;
+        }
+    }
+
+    function mod(uint256 a, uint256 b) public returns (uint256) {
+        // Does not disable div by zero check
+        unchecked {
+            return a % b;
+        }
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// div(uint256,uint256): 7, 2 -> 3
+// div(uint256,uint256): 7, 0 -> FAILURE # throws #
+// mod(uint256,uint256): 7, 2 -> 1
+// mod(uint256,uint256): 7, 0 -> FAILURE # throws #

test/libsolidity/syntaxTests/unchecked/unchecked_for_header.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f() public pure {
+        for (unchecked { uint x = 2 }; x < 2; x ++) {
+
+        }
+    }
+}
+// ----
+// ParserError 6933: (57-66): Expected primary expression.

test/libsolidity/syntaxTests/unchecked/unchecked_function_body.sol

@@ -0,0 +1,3 @@
+function f() pure returns (uint) unchecked {}
+// ----
+// ParserError 5296: (33-42): "unchecked" blocks can only be used inside regular blocks.

test/libsolidity/syntaxTests/unchecked/unchecked_modifier.sol

@@ -0,0 +1,5 @@
+contract C {
+    modifier m() { unchecked { _; } }
+}
+// ----
+// SyntaxError 2573: (44-45): The placeholder statement "_" cannot be used inside an "unchecked" block.

test/libsolidity/syntaxTests/unchecked/unchecked_nested.sol

@@ -0,0 +1,11 @@
+contract C {
+    function f() public pure {
+        unchecked {
+            unchecked {
+                uint x = 2 + 3;
+            }
+        }
+    }
+}
+// ----
+// SyntaxError 1941: (76-133): "unchecked" blocks cannot be nested.

test/libsolidity/syntaxTests/unchecked/unchecked_post_for.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f() public pure {
+        for (uint x = 2; x < 2; unchecked { x ++; }) {
+
+        }
+    }
+}
+// ----
+// ParserError 6933: (76-85): Expected primary expression.

test/libsolidity/syntaxTests/unchecked/unchecked_vardecl.sol

@@ -0,0 +1,8 @@
+contract C {
+    uint x = unchecked { f() + 2 }
+    function f() public pure returns (uint) {
+        return 4;
+    }
+}
+// ----
+// ParserError 6933: (26-35): Expected primary expression.

test/libsolidity/syntaxTests/unchecked/unchecked_while_body.sol

@@ -0,0 +1,9 @@
+contract C {
+    function f() public pure {
+        while (true) unchecked {
+
+        }
+    }
+}
+// ----
+// ParserError 5296: (65-74): "unchecked" blocks can only be used inside regular blocks.